<p>Our study comparatively evaluates seven alternative supplementary cementitious materials (SCMs) in their market-available form: diatomite, metakaolin, moler, waste expanded perlite, trass, wollastonite, and zeolite. The SCMs were used as a replacement for 10 wt% and 25 wt% of CEM I 42.5 R. Their effects on fresh and hardened cement composites were investigated by analyzing their impact on water demand, setting time, strength, porosity, shrinkage, heat evolution, and phase composition. At 10 wt% replacement, all SCMs met the EN 197–1 strength requirements. At 25 wt% replacement, only metakaolin, moler, trass, and zeolite maintained the required 28-day strength, demonstrating their better suitability for high-level clinker substitution. High-surface-area, porous SCMs increase water demand and mortar porosity. Therefore, at higher dosages, the use of superplasticizer is required. However, compressive strength showed no clear correlation with cumulative porosity or critical pore diameter, indicating that pore volume alone does not control mechanical performance. Most SCMs reduce drying shrinkage. Calorimetric and phase analyses confirmed that the SCMs alter hydration kinetics and sulfate–aluminate balance in the system. Higher Al₂O₃ content and specific surface area generally increase aluminate peak intensity and accelerate its occurrence. Reactive SCMs promote carboaluminate and C-(A)-S–H formation, with metakaolin, zeolite, and moler exhibiting the highest pozzolanic activity. Overall, the selected high-reactivity SCMs show great potential as components of sustainable, low-carbon blended cements.</p>